Use OS_CONFIG_FILES to populate metadata tree

Building on the lower level interface, now we can start operating in
a way that will work smoothly with os-collect-config.

This makes apply_config.read_config redundant, so it is removed.

We now parse the environment variable OS_CONFIG_FILES, as defined by
os-collect-config's documentation. The colon separated list is used
to determine the json config files to parse and merge.

Change-Id: I2a2eb7c4d2a3d06a73258cadcae4e7fe4bbb0096